XPS/NEXAFS spectroscopic and conductance studies of glycine on AlGaN/GaN transistor devices

摘要

Highlights•Glycine can form a dipole which affects the response of GaN/AlGaN transistors.•XPS is used to assess coverage and chemical nature of Glycine on the surface.•NEXAFS is used to understand the molecular orientation and binding mechanism.•In situ conductance measurements were conducted with the NEXAFS/XPS.AbstractWe report on a study using a combination of XPS/NEXAFS and conductivity measurements to develop a fundamental understanding of how dipolar molecules interact with the heterostructure device surface and affect the device conductivity of AlGaN/GaN heterostructure-based transistors. In such structures, which are increasingly being investigated for chemical and biological sensing, a 2-dimensional electron gas spontaneously forms at the layer interface that is sensitive to the charge characteristics of the exposed surface. Glycine, chosen for this study because it is the simplest of the amino acids and is known to form a zwitterionic configuration when stabilized through intermolecular interactions, was evaporated under ultra-high vacuum conditions onto the device surface and subsequently both XPS/NEXAFS and conductivity measurements were conducted. NEXAFS spectra show a preferential orientation for the Glycine molecules on the surface and evidence for both neutral and zwitterionic species on the surface. In situ conductivity measurements suggest that the negatively charged carboxylate group is closest to the surface. These results are a unique and pivotal contribution to the previous and at times conflicting literature on the zwitterionic nature of Glycine.